The radioactive liquid waste produced in the pressurized light water reactor has been treated in an evaporator or an ion-exchange resin bed. The evaporation method employing an evaporator, where the liquid waste is concentrated to reduce the amount of the waste, has advantages that: the distilled water can be recycled and the outflow of the radioactive material to the environment can be minimized.
The evaporation method, however, has been known to cause various difficult problems due to boric acid contained in the liquid waste. For example, since highly concentrated boric acid precipitates impair the evaporator, the maximum concentration of boric acid has to be maintained within 12 wt % of the liquid waste, which has been one of obstructive factors in reducing the volume of the liquid waste. In addition, the method essentially requires extra energy since the concentrated liquid waste is stored in the concentrate tank at an insulating temperature of 70.degree. C., and boric acid prolongs and interferes with curing of the cement, and the borated free water which remains unbound to the cement, corrodes the steel drums, which also plays an obstructive role in safe disposal, storage and treatment of the drums containing the waste.
Under the circumstances, a variety of methods by which boric acid can be separated from the liquid waste, and then recycled or put into disposal, have been developed in the art, e.g., solvent extraction, precipitation, ion-exchange, and membrane separation.
The solvent extraction method by which boric acid is extracted with solvents such as 2-ethylhexanediol-1,3 (EHD), 2,2-ethylbutylpropanediol (EBPD) or an organic solvent mixture of 25% (v/v) of isooxanol and 75% (v/v) of xylene containing 0.4M EBPD, has been commonly used in the art; however, it has shortcomings of a complex installation, an intricate and expensive operation, and an uneasy treatment of by-products.
The precipitation method, which is categorized into two methods, i.e., a method by which every positively charged materials except boric acid are precipitated and another method by which only boric acid is precipitated, has been known to have an advantage of a simple modification of the system. However, the method also has serious difficulties that: a large quantity of precipitant is required in the treatment; and, control of the radioactive precipitate accompanied by frequent co-precipitation is very inefficient.
The ion-exchange method, which comprises the steps of removing anions contained in the waste and collecting boric acid through a recycling operation after adsorbing boric acid to the resin, has a merit that the operation and modification of the system are very simple and easy. However, the method also has shortcomings that: the efficiency of the resin is changeable depended on the chemical properties of the waste; and, the treatment of contaminated resin is very difficult.
The membrane extraction method employing the reverse osmosis and the electrodialysis techniques has a merit that the extracted boric acid can be recycled without any further treatment. However, the reverse osmosis technique costs high for maintenance and repair, since it operates under a high pressure; and, the electrodialysis technique has a difficulty in installation of the equipment and also produces inflammable gases.
Accordingly, the prior art methods for boric acid separation have been proven to have various difficult problems and limitations for their practical and industrial application.